SEVEN-BAND SURFACE-MOUNT LOOP
ANTENNA WITH A CAPACITIVELY
COUPLED FEED FOR MOBILE PHONE
APPLICATION
Wei-Yu Li and Kin-Lu Wong
Department of Electrical Engineering, National Sun Yat-Sen
University, Kaohsiung 804, Taiwan; Corresponding author:
liwy@ema.ee.nsysu.edu.tw
Received 14 August 2008
Figure 6
Simulated and measured axial ratio against frequency
conical beam patterns was characterized and verified with measurements and shown to have acceptable axial ratio. The present
antenna design can be used as a good candidate for applications in
short-range wireless communication systems, particularly wireless
sensor networks, where the conical beam will give advantages of
energy saving and reduced co-channel interference. Other applications could include indoor WLAN antennas for mounting on
ceilings or other horizontal surfaces.
REFERENCES
1. Y.J. Guo, A. Paez, R.A. Sadeghzadeh, and S.K. Barton, A circular
patch antenna for radio LANs, IEEE Trans Antennas Propag 2 (1997)
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2. R.A. Abd-Alhameed, N.J. McEwan, E.M. Ibrahim, and P.S. Excell, A
new design of horizontally-polarised and dual-polarized uniplanar
antennas for HIPERLAN, IEEE Trans Antennas Propag 51 (2003)
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3. H. Kawakami, G. Sato, and R. Wakabayashi, Research on circularly
polarized conical-beam antennas, IEEE Antennas Propag Mag 39
(1997) 27–39.
4. M. Krairiksh, C. Phongcharoenpanich, K. Meksamoot, and J.-I.
Takada, A circularly polarized conical beam spherical slot array antenna, Int J Electr 86 (1999) 815– 823.
5. C. Phongcharoenpanich, T. Lertwiriyaprapa, S. Lamultree, P. Wounchoum, S. Kosulvit, and M. Krairiksh, Characteristics of a helical array
antenna radiating circularly polarized conical beam, IEEE Antennas
Propag Soc Int Symp 4 (2001) 557–560.
6. F. Ares, G. Franceschetti, L. Mosig, S. Vaccaro, J. Vassal’lo, and E.
Moreno, Satellite communication with moving vehicles on earth: two
prototype circular array antennas, Microwave Opt Technol Lett 39
(2003) 14 –16.
7. K.L. Lau and K.M. Luk, A wideband circularly polarized conicalbeam patch antenna, IEEE Trans Antennas Propag 54 (2006) 1591–
1594.
8. K.-L. Wong, Compact and broadband microstrip antennas, John Wiley
& Sons, New York, 2002.
9. R.A. Abd-Alhameed, K. Khalil, P.S. Excell, M.M. Ibrahim, and R.
Alias, Stripline-fed circular-polarised microstrip antennas for satellite
communications, Twelfth International Conference on Antennas and
Propagation (ICAP 2003), University of Exeter, UK, Vol. 2, 2003, pp.
635– 638.
10. Computer Simulation Technology Corporation, CST Microwave Studio, Version 5.0, Germany.
© 2008 Wiley Periodicals, Inc.
DOI 10.1002/mop
ABSTRACT: A surface-mount loop antenna with a capacitively coupled feed capable of seven-band operation in the mobile phone is presented. The capacitively coupled feed successfully excites the 0.5-, 1.0-,
and 1.5-wavelength modes of the loop antenna. Further, the presence of
the capacitively coupled feed leads to a new loop path, whose length is
slightly less than that of the original loop strip. The 0.5-, 1.0-, and 1.5wavelength modes of the new loop path are also excited with good impedance matching. The two 0.5-wavelength modes of the new loop path
and original loop strip form a wide bandwidth of 310 MHz for the antenna’s lower band to cover GSM850/900 operation. The other modes of
the two loops form a very wide bandwidth of larger than 1 GHz for the
antenna’s upper band to cover GSM1800/1900/UMTS/WLAN/WiMAX
operation. With seven-band operation achieved, the proposed loop antenna only occupies a small volume of 60 ⫻ 10 ⫻ 3 mm3 (1.8 cm3), and
owing to its thin thickness of 3 mm only, and the antenna is very promising for thin mobile phone applications. © 2008 Wiley Periodicals, Inc.
Microwave Opt Technol Lett 51: 81– 88, 2009; Published online in
Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.
23955
Key words: internal mobile phone antennas; loop antennas; surfacemount antennas; capacitively coupled feed; multiband operation
1. INTRODUCTION
Recently, a variety of multiband loop antennas for wireless wide
area network (WWAN) operation in the mobile phone have been
demonstrated [1–7]. These loop antennas are attractive for mobile
phone applications in part, because their excited surface current
paths are in a closed form, which is different from the conventional
mobile phone antennas such as the monopoles or planar inverted-F
antennas (PIFAs), whose excited surface current paths are in an
open form [8]. Hence, the possible coupling between the loop
antenna and the system ground plane of the mobile phone can be
much smaller than that of the conventional mobile phone antennas.
In this case, the variations in the dimensions of the system ground
plane will show smaller effects on the performances of the loop
antenna.
For the reported multiband loop antennas for mobile phone
applications [1–7], most of them use a direct feed, and the obtained
operating bands cover several of the GSM850 (824 – 894 MHz),
GSM900 (890 –960 MHz), GSM1800 (1710 –1880 MHz),
GSM1900 (1850 –1990 MHz), and UMTS (1920 –2170 MHz)
bands for WWAN operation. In this article, we present a surfacemount loop antenna with a capacitively coupled feed for achieving
seven-band operation covering all the five operating bands for
WWAN operation, the 2.4-GHz band (2400 –2484 MHz) for wireless local area network (WLAN) operation, and the 2.5-GHz band
(2500 –2690 MHz) for worldwide interoperability for microwave
access (WiMAX) operation [9 –15]. That is, WWAN/WLAN/
WiMAX multinetwork operation can be achieved for the proposed
antenna.
The capacitively coupled feed applied in the proposed loop
antenna comprises a coupling strip and a tuning strip, which is
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009
81
different from the conventional capacitive feed that has been
studied for internal mobile phone antennas [5, 16 –18]. The conventional capacitive feed mainly has a coupling strip or coupling
portion only, and with the capacitive feed, widened bandwidth in
the lower band (900 MHz band) of the antenna [16] or reduced size
of the antenna [17, 18] has been demonstrated. In [5], by using a
coupling stub as the capacitive feed, two wide operating bands are
achieved to cover GSM850/900/1800/1900 quad-band operation
for the loop antenna with an occupied volume of 3.4 cm3.
In the proposed design, the loop antenna is formed by mounting
a meandered loop strip onto the surfaces of a foam base of 60 ⫻
10 ⫻ 3 mm3 (1.8 cm3) only. The antenna is then surface-mounted
to the system circuit board of the mobile phone on top of the
capacitively coupled feed printed on the system circuit board.
Through the capacitive coupling of the coupling strip in the capacitively coupled feed, the 0.5-, 1.0-, and 1.5-wavelength modes
of the surface-mount loop antenna can be successfully excited. In
addition, the capacitively coupled feed can generate a new loop
path, whose length is slightly less than that of the original loop
strip, and the 0.5-, 1.0-, and 1.5-wavelength modes of the new loop
path can also be excited. The two 0.5-wavelength modes of the
new loop path and original loop strip form a wide lower band for
the antenna to cover GSM850/900 operation. The 1.0- and 1.5wavelength modes of the new loop path and original loop strip can
also be formed into a wide upper band for the antenna by further
selecting a proper length of the tuning strip in the capacitively
coupled feed, and the obtained upper band can cover GSM1800/
1900/UMTS/WLAN/WiMAX operation. The proposed loop antenna is hence capable of seven-band operation in three different
wireless networks. The proposed antenna with the capacitively
coupled feed is studied in the article. Experimental and simulation
results of the constructed prototype are presented and discussed.
2. ANTENNA DESIGN
Figure 1(a) shows the geometry of the proposed seven-band surface-mount loop antenna with the capacitively coupled feed. The
loop antenna is a surface-mountable element, and it is to be
mounted on the front side of the top no-ground portion (area 60 ⫻
10 mm2) of the system circuit board of the mobile phone (a
0.8-mm-thick FR4 substrate of area 60 ⫻ 110 mm2 used here) for
practical applications. A ground plane of 60 ⫻ 100 mm2 is printed
on the back side of the FR4 substrate as the system ground plane
of the mobile phone. The selected dimensions of the system circuit
board and the ground plane are reasonable for general smart
phones or PDA phones [19, 20].
The loop antenna is obtained by attaching a meandered loop
strip onto the surfaces of a thin foam base of volume 60 ⫻ 10 ⫻
3 mm3 (1.8 cm3). The meandering of the loop strip is to achieve a
longer length (total length about 265 mm here, starting from point
A, then through points C and D, to point B) on the fixed surfaces
of the foam base. Detailed dimensions of the loop strip in its planar
structure are given in Figure 1(b). The two ends (point A and point
B) of the loop strips are grounded to the top edge of the system
ground plane, and hence a closed loop path is formed. The loop
strip has a narrow width of 0.5 mm, except at the widened section
C៮ D of length 15 mm and width 2.5 mm. The widened section is
centered on top of the coupling strip in the capacitively coupled
feed printed on the back side of the top no-ground portion.
Through the capacitive coupling between the widened section and
the coupling strip, the 0.5-, 1.0-, and 1.5-wavelength modes of the
loop strip at about 0.75, 1.7, and 2.3 GHz can be excited with good
impedance matching. The good excitation is easily controlled by
adjusting the dimensions of the coupling strip whose preferred
82
Figure 1 (a) Geometry of the proposed surface-mount loop antenna with
the capacitively coupled feed for seven-band operation in the mobile
phone. (b) Dimensions of the antenna in its planar structure. [Color figure
can be viewed in the online issue, which is available at www.interscience.
wiley.com]
width (w) is 0.5 mm and length (k) is 13.5 mm. Their detailed
effects will be analyzed in Figure 5 in the next section.
In addition to the coupling strip, there is a tuning strip of length
(t) 20 mm and width 2 mm in the capacitively coupled feed. The
front end (point E) of the capacitively coupled feed is the antenna’s
feeding point, which is connected to a 50-⍀ microstrip feedline
printed on the back side of the system circuit board. It can be seen
that, with the presence of the capacitively coupled feed, a new loop
៮ D, to point B is
path starting from point E, through section C
formed, whose length is about 255 mm, slightly shorter than that
of the original loop strip (loop ACDB or loop 1). The 0.5-, 1.0-,
and 1.5-wavelength modes of the new loop path (loop ECDB or
loop 2) at about 1.0, 2.1, and 2.7 GHz can also be excited with
good impedance matching by selecting a proper length of the
tuning strip (the preferred length t is 20 mm here). Detailed effects
of the length t are studied in Figure 6.
With good excitation of the resonant modes of the two loops
(loops 1 and 2), two wide operating bands for the antenna are
achieved. The two 0.5-wavelength modes of loops 1 and 2 at about
0.75 and 1.0 GHz are formed into a wide lower band for the
antenna to easily cover GSM850/900 operation. The 1.0- and
1.5-wavelength modes of the two loops at about 1.7, 2.1, 2.3, and
2.7 GHz are also formed into a very wide band for the antenna’s
upper band to cover GSM1800/1900/UMTS/WLAN/WiMAX op-
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009
DOI 10.1002/mop
be reduced to be 1.2 cm3 only. Detailed effects of the thickness h
are discussed in Figure 7 in the next section.
3. RESULTS AND DISCUSSION
Figure 2 Measured and simulated return loss for the proposed antenna.
[Color figure can be viewed in the online issue, which is available at
www.interscience.wiley.com]
eration. A seven-band operation is hence achieved for the proposed
antenna with a volume of 1.8 cm3 only. In addition, the thickness
h of the proposed loop antenna or the thickness of the foam base
can still be decreased from 3 to 2 mm, without large effects on the
antenna performances. In this case, the antenna volume can further
The proposed loop antenna shown in Figure 1 was fabricated and
tested. Figure 2 shows the measured and simulated return loss for
the constructed prototype. The simulated results are obtained using
Ansoft HFSS [21]. From the results, good agreement between the
measurement and simulation is seen. It can be seen that the
antenna’s lower band formed by two resonances (0.5-wavelength
modes) has a wide bandwidth of 310 MHz ranging from 700 to
1010 MHz (3:1 VSWR or 6-dB return loss), allowing it to easily
cover GSM850/900 operation. The antenna’s upper band mainly
formed by the 1.0- and 1.5-wavelength modes of loops 1 and 2
(loops ACDB and ECDB) shows a very large bandwidth of 1110
MHz (1700 –2810 MHz) and easily covers GSM1800/1900/
UMTS/WLAN/WiMAX operation. In addition, the impedance
matching for frequencies over the WLAN (2400 –2484 MHz) and
WiMAX (2500 –2690 MHz) bands is better than 10-dB return loss.
To study the excited resonant modes more clearly, a comparison of the simulated return loss of the proposed antenna and the
reference antenna (the corresponding loop antenna with a conven-
1,000
800
600
400
200
0
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-400
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-1,000
5E+008
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600
400
200
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-200
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Figure 3 (a) Simulated return loss of the proposed and reference antennas. (b) Dimensions of the reference antenna in its planar structure; the
reference antenna occupies the same volume as the proposed antenna.
[Color figure can be viewed in the online issue, which is available at
www.interscience.wiley.com]
DOI 10.1002/mop
Figure 4 Simulated input impedance for (a) the proposed antenna and
(b) the reference antenna in Figure 3. [Color figure can be viewed in the
online issue, which is available at www.interscience.wiley.com]
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009
83
tional feed) is shown in Figure 3(a). Dimensions of the reference
antenna in its planar structure are shown in Figure 3(b), and the
reference antenna occupies the same volume as the proposed
antenna. Note that the excited modes of 0.5␭ mode 1, 1.0␭ mode
1, and 1.5␭ mode 1 indicated in the figure at about 0.8, 1.7, and 2.3
GHz are contributed by loop 1 (loop ACDB), while the 0.5␭ mode
2, 1.0␭ mode 2, and 1.5␭ mode 2 at about 1.05, 2.1, and 2.7 GHz
are controlled by loop 2 (loop ECDB). Also, as compared to the
reference antenna, the proposed antenna shows much wider lower
and upper bands for covering GSM850/900 and GSM1800/1900/
UMTS/WLAN/WiMAX operations.
To show the effects of the capacitively coupled feed more
clearly, a comparison of the simulated input impedance for the
proposed and reference antennas is also shown in Figures 4(a) and
4(b). The indicated 0.5␭ resonance 1, 1.0␭ resonance 1, and 1.5␭
resonance 1 are zero reactance controlled by loop 1, and near these
resonances, the three resonant modes of 0.5␭ mode 1, 1.0␭ mode
1, and 1.5␭ mode 1 shown in Figure 3(a) are excited. Similarly, the
0.5␭ resonance 2, 1.0␭ resonance 2, and 1.5␭ resonance 2 are zero
reactance controlled by loop 2, which leads to the excitation of
three related resonant modes controlled by loop 2 shown in Figure
3(a). Note that, in addition to these resonances generated owing to
Figure 5 Simulated return loss as a function of (a) the width w and (b)
the length k of the coupling strip in the capacitively coupled feed. Other
dimensions are the same as given in Figure 1. [Color figure can be viewed
in the online issue, which is available at www.interscience.wiley.com]
84
Figure 6 Simulated return loss as a function of the length t of the tuning
strip in the capacitively coupled feed. Other dimensions are the same as
given in Figure 1. [Color figure can be viewed in the online issue, which
is available at www.interscience.wiley.com]
the use of the capacitively coupled feed, the input resistance level
(real part of the input impedance) is also much smaller than that of
the reference antenna shown in Figure 4(b). This makes the input
resistance level much closer to 50 ⍀. The variations in the input
reactance (imaginary part of the input impedance) are also found to
be much smaller for the proposed antenna. These attractive features result in good excitation of the 0.5-, 1.0-, and 1.5-wavelength
modes of loops 1 and 2 in the proposed antenna. Also note that the
resonance occurring at about 1.35 GHz in both Figures 4(a) and
4(b) is mainly controlled by the system ground plane and does not
contribute to the desired operating bands for the proposed antenna.
Effects of the width w and length k of the coupling strip are
studied Figure 5. The simulated return loss for the width w varied
from 0.5 to 1.5 mm is shown in Figure 5(a); other dimensions are
the same as given in Figure 1. Results indicate that the dualresonance excitation (two 0.5-wavelength modes of loops 1 and 2)
of the antenna’s lower band is achieved by using a small width of
0.5 mm. The effects on the antenna’s upper band, however, are
very small. Simulated results for the length k varied from 9.5 to
13.5 mm are presented in Figure 5(b), with the width w fixed as 0.5
mm. Results show that the three resonant modes contributed by
Figure 7 Simulated return loss as a function of the thickness h of the
antenna. Other dimensions are the same as given in Figure 1. [Color figure
can be viewed in the online issue, which is available at www.
interscience.wiley.com]
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009
DOI 10.1002/mop
Figure 8 Measured radiation patterns at (a) 859 MHz and (b) 920 MHz for the antenna. [Color figure can be viewed in the online issue, which is available
at www.interscience.wiley.com]
loop 2 (loop ECDB) are shifted to higher frequencies when the
length k decreases, while those controlled by loop 1 (loop ACDB)
are almost not affected. This is largely because the variations in the
length k of the coupling strip can lead to some variations in the
capacitive coupling, which in turn results in the effective length
variations of loop 2.
Effects of the length t of the tuning strip in the capacitively
coupled feed are studied in Figure 6. Simulated results of the return
loss for the length t varied from 16 to 24 mm are presented, while
other parameters are the same as given in Figure 1. Results show
that the two modes in the antenna’s lower band are almost not
affected; however, the 1.0- and 1.5-wavelength modes of loop 2
show large variations. This is largely because the length of the
tuning strip is very small as compared to that at 1 GHz, and hence
relatively very small effects on the antenna’s lower band can be
expected. On the other hand, by adjusting the length t of the tuning
DOI 10.1002/mop
strip, good excitation of the 1.0- and 1.5-wavelength modes of loop
2 can be obtained.
Figure 7 shows the simulated return loss as a function of the
thickness h of the loop antenna or the foam base. Results for h
varied from 2 to 4 mm with other parameters fixed, as given in
Figure 1, are presented. Small effects on the impedance matching
of the antenna’s lower and upper bands are seen, except that the
impedance matching level around 2.2 GHz is slightly smaller than
the 10-dB return loss for the case of h ⫽ 2 mm. However, the
obtained operating bands for the case of h ⫽ 2 mm can still cover
the desired seven-band operation in this study. This makes it
possible to further reduce the thickness of the proposed antenna to
2 mm only, which makes the antenna attractive for applications in
thin mobile phones [20, 22]. Also, with a thickness of 2 mm only,
the occupied volume of the antenna can be reduced to 1.2 cm2
(60 ⫻ 10 ⫻ 2 mm3) only.
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85
Figure 9 Measured radiation patterns at (a) 1795 MHz, (b) 1920 MHz, and (c) 2045 MHz for the antenna. [Color figure can be viewed in the online issue,
which is available at www.interscience.wiley.com]
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DOI 10.1002/mop
Radiation characteristics of the constructed prototype are also
studied. Figure 8 plots the measured radiation patterns at 859 and
920 MHz. It can be seen that the radiation patterns at 859 and 920
MHz show monopole-like patterns and are similar to each other,
which indicates that stable radiation characteristics are obtained
over the antenna’s lower band.
Figure 9 plots the measured radiation patterns at 1795, 1920,
and 2045 MHz, which are the central frequencies of GSM1800,
GSM1900, and UMTS bands, respectively. Figure 10 plots the
measured radiation patterns at 2442 and 2595 MHz, central frequencies of the 2.4 GHz WLAN band and the 2.5 GHz WiMAX
band. It is also seen that the radiation pattern at 1795, 1920, and
2045 MHz are similar to each other. This is reasonable since the
operating bands of GSM1800/1900/UMTS are mainly provided by
two 1.0-wavelength modes of loops 1 and 2. For the radiation
patterns at 2442 and 2595 MHz, they are also similar to each other.
Again, this is because the operating bands of WLAN/WiMAX are
mainly covered by two 1.5-wavelength modes of loops 1 and 2.
Figure 11 shows the measured antenna gain and simulated
radiation efficiency. In Figure 11(a), over the GSM850/900 bands,
the antenna gain is about 0.5–1.3 dBi, and the efficiency is larger
than 70%. Over the GSM1800/1900/UMTS bands shown in Figure
11(b), the antenna gain is about 0.5–2.7 dBi, and the efficiency is
about 65– 85%. While over the 2.4 GHz WLAN band, the antenna
gain is about 1.5 dBi and the efficiency is about 87%. Over the
2.5-GHz WiMAX band, the antenna gain is about 1.3–2.8 dBi and
the efficiency is about 75– 85%.
4. CONCLUSION
A novel seven-band surface-mount loop antenna with a capacitively coupled feed has been proposed for mobile phone applications. The capacitively coupled feed provides an additional loop
Figure 10 Measured radiation patterns at (a) 2442 MHz and (b) 2595 MHz for the antenna. [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com]
DOI 10.1002/mop
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009
87
Figure 11 Measured antenna gain and simulated radiation efficiency
over (a) GSM850/900 bands and (b) GSM1800/1900/UMTS/WLAN/
WiMAX bands. [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com]
path to the original loop strip, and excites the 0.5-, 1.0-, and
1.5-wavelength modes of the two loops successfully. These excited resonant modes are formed into two wide operating bands for
the antenna’s lower and upper bands to cover GSM850/900 and
GSM1800/1900/UMTS/WLAN/WiMAX operations. Good radiation characteristics over the seven operating bands have also been
obtained. In addition, the antenna occupies a small volume of 60 ⫻
10 ⫻ 3 mm3 (1.8 cm3) only, with a thin thickness of 3 mm. It is
also promising to reduce the thickness to 2 mm only, which can
further decrease the occupied antenna volume to be 1.2 cm3. The
small volume and thin thickness make the proposed antenna very
suitable for thin mobile phone applications.
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© 2008 Wiley Periodicals, Inc.
DESIGN OF CPW-FED MONOPOLE
UWB ANTENNA WITH A NOVEL
NOTCHED GROUND
Xin Zhang, Wei Wu, Ze-Hong Yan, Jun-Bo Jiang, and
Yue Song
National Laboratory of Antennas and Microwave Technology, Xidian
University, Xi’an, Shaanxi 710071, People’s Republic of China;
Corresponding author: zx8381@163.com
Received 11 May 2008
ABSTRACT: A novel CPW-fed monopole UWB antenna is presented in
this article. To increase the impedance bandwidth, a notched ground is
introduced. The parameters and the characteristics of the antenna are
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009
DOI 10.1002/mop